Exposure control method for digital camera

ABSTRACT

An apparatus and method for controlling exposure of a digital camera with high precision by setting a proper exposure amount by emitting auxiliary light during auto-focusing, detecting the luminance level of an object, and carrying out an exposure controlling operation. The apparatus detects the luminance level of an object in an AE frame under the outside light prior to emitting the auxiliary light, and detects again the luminance level of the object in an AF frame under the auxiliary light after emitting the auxiliary light. The apparatus further performs an exposure controlling operation based on the luminance level in the AE frame when it has been determined that the exposure amount of the AF frame has increased markedly in a determining step, or based on the luminance level in the AF frame when it has been determined that the exposure amount of the AF frame has not increased markedly in the determining step.

TECHNICAL FIELD

The present invention relates to an apparatus and method for controllingexposure of a digital camera comprising an auxiliary light emitter fortemporarily illuminating an object with auto-focus auxiliary light onlyduring auto-focusing in order to provide light when the luminance levelis not sufficient.

BACKGROUND ART

In general, in a digital camera, exposure of an object is controlled bysetting the opening ratio of an aperture that is provided at a lens, theelectronic shuttering amount of a CCD, which is an imaging device, andthe gain of an imaging signal output from the CCD. Many digital camerasuse a hill-climbing auto-focusing device as an auto-focusing(hereinafter abbreviated as “AF”) device.

Here, auto-focusing means to automatically bring a taking lens intofocus with an object. In auto-focusing, the distance to the object ismeasured to obtain focusing. Hill-climbing auto-focusing provides theprinciple of measuring distance in auto-focusing in which the locationof a lens where a high-frequency component having a luminance leveldetected by the CCD becomes a maximum is the location of light focus.

Therefore, when an object in a dark background does not have sufficientcontrast, the position of the object can no longer be determined, sothat the location of light focus cannot be adjusted. To overcome thisproblem, when there is not sufficient contrast because the luminancelevel of the object in a dark background is low, auto-focusing iscarried out. During the auto-focusing, an exposure controlling operationwhich increases the gain of an imaging signal is carried out. However,when the luminance level is very low because the object is at a placewhere there is very little light around it, such as in the dark, theluminance level remains insufficient even if the exposure controllingoperation which increases the gain of an imaging signal is carried out,so that it may not be possible to increase the precision of theauto-focusing. To overcome this problem, in recent digital cameras, anobject is illuminated with AF auxiliary light in order to provide lightwhen the luminance level is insufficient. In such digital cameras, anauxiliary light emitter for emitting AF auxiliary light is built in thecamera body, or is built in a flash unit for illuminating an object toproperly expose it when it is photographed in a dark environment oragainst the sun and is mounted to the camera body when necessary.

Here, the amount of exposure control (opening ratio of an aperture,electronic shuttering amount of a CCD, gain of an imaging signal) isdetermined by comparing the luminance level of the object with a targetvalue that is stored in RAM, which is a memory that needs to besequentially subjected to a writing operation, and feeding back theresult of the comparison. Therefore, when the luminance level of theobject is not a proper level, the amount of exposure control alsobecomes an improper amount.

In related digital cameras, although the amount of exposure control isupdated based on the luminance level while the object is being monitoredbefore photographing it, the amount of exposure control is not updatedduring auto-focusing. In the case where auxiliary light is emitted, thatis, the background is bright, when the luminance level of the object atthe time of monitoring and that at the time of auto-focusing are thesame, no problems arise even if the amount of exposure control is notupdated. However, in the case where the luminance level of the object atthe time of monitoring and that at the time of auto-focusing are not thesame, when the luminance level of the object at the time of monitoringis used, the exposure is no longer controlled by a proper value. Forthis reason, in related digital cameras using AF auxiliary light, theprecision of auto-focusing cannot be increased. In order to mitigatethis problem and perform a proper exposure controlling operation evenafter emission of auxiliary light, it is necessary to compute the amountof exposure control again by detecting again the luminance level of theobject after the emission of auxiliary light.

As mentioned above, when photographing an object with an AF camera usinga hill-climbing method, in order to focus an image when it is beingphotographed, the object needs to have sufficient contrast. There aremany situations where the object does not have sufficient contrast, suchas when the outline of the object is not clear. When the object cannothave sufficient contrast because its luminance level is low, the imagecan be focused by increasing the luminance level. In other words, duringauto-focusing, an exposure controlling operation which increases thegain of an imaging signal compared to that during monitoring is carriedout. When sufficient contrast still cannot be provided due toinsufficient luminance level even after this exposure controllingoperation has been carried out, the object is illuminated with AFauxiliary light of, for example, an LED, to increase the luminance levelof the object for achieving focus. In a camera including an auxiliarylight emitter for emitting AF auxiliary light, when the amount of lightof the place where the object exists is determined as being low duringfocusing, a controlling operation is carried out so that auxiliary lightis emitted.

In general, the operation modes to the time of recording an image in adigital camera changes from a monitoring mode to a scan AF mode to aphotographic mode in that order. Here, the monitoring mode is a mode inwhich data of an image is displayed on a display section without storingit on a storage medium (in DRAM) to monitor an object.

The scan AF mode is a mode for achieving focus before recording animage. For example, in the scan AF mode, a shutter button is pressedhalfway. Here, in order to make it easier to perform AF, the aperture isbrought close to an open state and the focal depth is made small. Whenthe luminance level of an object is low, an exposure controllingoperation which increases the gain of an imaging signal is carried outto obtain sufficient contrast. The photographic mode is a mode forrecording an image.

Conventionally, during monitoring mode, an exposure controllingoperation (opening ratio of an aperture, electronic shuttering amount ofa CCD, and gain of an imaging signal) is carried out for each framerate. However, when the operation mode of the camera changes from themonitoring mode to the scan AF mode, an exposure controlling operationis not carried out again in the scan AF mode. Therefore, when auxiliarylight is emitted, the amount of exposure control does not change eventhough the luminance level changes, so that the exposure when auxiliarylight is emitted is no longer carried out by a proper amount. Thisresults in a problem in that the precision of the auto-focusing isreduced.

The present invention has been achieved to overcome problems such asthose mentioned above, and has as its object the provision of a methodfor controlling exposure of a digital camera which, by controlling anexposure operation again even after emission of auxiliary light forchanging luminance, makes it possible to perform proper exposure evenwhen auxiliary light is emitted, so that the precision of auto-focusingis increased.

DISCLOSURE OF INVENTION

To this end, the present invention provides a method for controllingexposure of a digital camera comprising means for emitting auxiliarylight. The method comprises the steps of detecting the illuminance ofoutside light; determining whether or not the auxiliary light isrequired based on the detected illuminance of the outside light; priorto emitting the auxiliary light, detecting the luminance level of anobject in an AE frame in a monitoring mode under the outside light;after emitting the auxiliary light, detecting again the luminance levelof the object in an AF frame in a scan AF mode under the auxiliarylight; determining whether or not the exposure amount of the AF framehas increased markedly than the exposure amount of the AE frame; andwhen it has been determined that the exposure amount of the AF frame hasincreased markedly in the determining step, performing an exposurecontrolling operation based on the luminance level in the AE frame, or,when it has been determined that the exposure amount of the AF frame hasnot increased markedly in the determining step, performing an exposurecontrolling operation based on the luminance level in the AF frame.

In the present invention, the exposure controlling step comprisescontrolling the opening ratio of an aperture, the electronic shutteringamount of an imaging device, and the gain of an imaging signal outputfrom the imaging device.

In the method for controlling exposure of a digital camera of thepresent invention, when the ratio between the exposure amount after theemission of the auxiliary light and the exposure amount prior to theemission of the auxiliary light exceeds a predetermined threshold value,the exposure amount in the AF frame is limited.

According to the present invention, there is provided an exposurecontroller of a digital camera comprising means for emitting auxiliarylight, the exposure controller comprising means for detecting theilluminance of outside light; means for determining whether or not theauxiliary light is required based on the detected illuminance of theoutside light; means for detecting the luminance level of an object inan AE frame in a monitoring mode under the outside light prior toemitting the auxiliary light, means for detecting again the luminancelevel of the object in an AF frame in a scan AF mode under the auxiliarylight after emitting the auxiliary light; means for determining whetheror not the exposure amount of the AF frame has increased markedly thanthe exposure amount of the AE frame; and means for performing anexposure controlling operation based on the luminance level in the AEframe when it has been determined that the exposure amount of the AFframe has increased markedly in the determining step, or based on theluminance level in the AF frame when it has been determined that theexposure amount of the AF frame has not increased markedly in thedetermining step.

In the exposure controller of a digital camera of the present invention,the exposure controlling means controls the opening ratio of anaperture, the electronic shuttering amount of an imaging device, and thegain of an imaging signal output from the imaging device.

In the exposure controller of a digital camera, when the ratio betweenthe exposure amount after the emission of the auxiliary light and theexposure amount prior to the emission of the auxiliary light exceeds apredetermined threshold value, the exposure amount in the AF frame islimited.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the structure of a circuit of adigital camera control section used in an embodiment of the presentinvention.

FIG. 2 illustrates a timing of setting the exposure amount whenauxiliary light is emitted and not emitted.

FIG. 3 is a flowchart showing the flow of an exposure control operationin a digital camera.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereunder, a detailed description of one embodiment of a digital cameraof the present invention will be given with reference to the drawings.FIG. 1 is a block diagram showing the structure of a circuit of adigital camera control section used in an embodiment of the presentinvention.

As shown in FIG. 1, the digital camera of the embodiment comprises alens 1 for receiving light from an object, an aperture 2 for adjustingexposure, and a CCD 3, which is an imaging device, where a light imageof the object from the lens 1 is focused. A holding/gain control circuit4 which sample holds an imaging signal output from the CCD 3 and whichcontrols the gain of the imaging signal is connected to an outputterminal of the CCD 3. An A/D converter 5 for subjecting the imagingsignal whose gain has been controlled to analog-to-digital conversion isconnected to this holding/gain control circuit 4.

An image signal processing circuit 6 for variously processing theimaging signal is connected to an output terminal of the AD converter 5.A luminance level detector 7 for detecting as a luminance level anintegral value of a luminance signal from a predetermined image area andextracted from an image signal is connected to the image signalprocessing circuit 6.

A microcomputer 8 for controlling the entire operation of the digitalcamera comprises an exposure control circuit 9, a device control circuit10, and an operational circuit 14. The exposure control circuit 9 isconnected to an output terminal of the luminance level detector 7. Basedon the luminance level detected by the luminance level detector 7, theexposure control circuit 9 determines whether or not proper exposure isbeing carried out. Based on the obtained data on the result of thedetermination, the exposure control circuit 9 computes control dataregarding each device, that is, data on the opening ratio of theaperture 2, data on the electronic shuttering amount of the CCD 3, andgain data from the holding/gain control circuit 4. The exposure controlcircuit 9 has RAM (not shown). Based on the luminance level detected bythe luminance level detector 7, various pieces of data that are requiredwhen an exposure controlling operation is carried out are written toRAM. In addition, the various pieces of data are read out from RAM.

The device control circuit 10 for outputting a drive signal of eachdevice is connected to an output terminal 9 a of the exposure controlcircuit 9. An aperture driver 11 is connected to a first output terminal10 a of the device control circuit 10. The aperture 2 is such as to bedriven by an output signal from the aperture driver 11. A timinggeneration circuit 12 for outputting a timing signal that controls theelectronic shutter of the CCD 3 is connected to a second output terminal10 b of the device control circuit 10. The holding/gain control circuit4 is connected to a third output terminal 10 c of the device controlcircuit 10.

The operational circuit 14 is connected to an output terminal 9 b of theexposure control circuit 9. From the luminance level, that is, theintegral value of a luminance signal from a predetermined image area andextracted from an image signal, computed by the luminance level detector7, and the control data of each device that is computed based on theluminance level by the exposure control circuit 9, the operationalcircuit 14 makes a computation to determine whether or not emission ofAF auxiliary light is required. An LED 15 for emitting AF auxiliarylight is connected to an output terminal of the operational circuit 14,and emits light when necessary.

Next, a description of the exposure controlling operation of the digitalcamera of the present invention will be given based on theabove-described structure. In the embodiment, the exposure controllingoperation is considered as being carried out when the luminance level ofan object is changed by emitting auxiliary light in the scan AF mode.More specifically, the luminance level changes from monitoring toauto-focusing after emission of auxiliary light. Even at the time of theauto-focusing, the luminance level of the object is detected again (theCCD takes in light) for controlling exposure, thereby increasing theprecision of the auto-focusing. Auxiliary light is only emitted when theamount of light of the surrounding is small the luminance level is low,so that it is not emitted when the amount of light of the surrounding islarge.

FIG. 2 shows a sequence of the temporal flow of the exposure controllingoperation when auxiliary light is emitted.

FIG. 2 illustrates from the top (i) sequence of change in the operationmode of the camera, (ii) on and off states of the LED 15 for emittingauxiliary light, (iii) the CCD 3 takes in (detects) light, (iv)computation at the image signal processing circuit 6, and (v)computation of the exposure amount. Here, each number along thehorizontal axis represents frame rate numbers (hereinafter referred toas the “frame numbers”). As shown in FIG. 2, the frame number where thelast computation of the exposure amount is carried out in a monitoringmode a1 is 0.

First, detection frames of a liquid crystal display will be described.Since up to frame number 0 the computation of the exposure amount iscarried out in the monitoring mode a1, the luminance level is detectedat a luminance level detection frame (hereinafter referred to as the “AEframe”) that is such as to allow proper exposure during the monitoringmode in order to compute the exposure amount. Here, the AE frame refersto the area of the entire liquid crystal display. Since the computationof the exposure amount from frame number 1 onwards is carried out in ascan AF mode a2, ordinarily, the luminance level is detected at aluminance level detection frame (hereinafter referred to as the “AFframe”) that is such as to allow proper exposure during the scan AF modein order to compute the exposure amount. Here, the AF frame refers to anarea of a portion of the screen central portion of the entire area ofthe liquid crystal display.

When the camera operation mode changes from the monitoring mode a1 tothe scan AF mode a2, the detection frame changes from the AE frame tothe AF frame as mentioned above. This is because, when auto-focusing isto be carried out, it is necessary to increase the precision of theauto-focusing by previously providing a detection frame which is thoughtto have a high possibility of having an object in an angle of view andby detecting the luminance level inside the detection frame. Inaddition, when zooming is carried out, also considering, for example,the range of illumination of the auxiliary light, the AF frame changesto an optimal detection frame for carrying out auto-focusing. However,in the case where the exposure controlling operation is carried out by asuitable exposure amount of the AF frame, when the luminance level of animage increases markedly compared to the luminance level in themonitoring mode, the AE frame is used as the detection frame as in themonitoring mode.

To frame number 1 where the exposure amount is computed immediatelyafter the operation mode has changed to the scan AF mode, operationflows as indicated by arrow A shown in FIG. 2, that is, (iii) the CCD 3takes in light, (iv) computation is carried out by the image signalprocessing circuit 6, and (v) by the operation flow indicated by arrowA, the exposure amount is computed based on the luminance level at framenumber 1 where the CCD has taken in light {(1) the exposure amount isdetermined}. For this reason, the exposure amount is computed by theluminance level before auxiliary light is emitted (when auxiliary lightis not emitted). In other words, since, at frame number 1 where theexposure amount is computed, the operation mode has already changed tothe AF mode, by selecting the AF frame, the luminance level of theobject is detected to compute the exposure amount.

The exposure amount computed at frame number 1 is reflected when the CCDhas taken in light at frame number 4 (which is shaded) by an operationflow indicated by arrow B shown in FIG. 2 {(2) the exposure amount isreflected}. When the CCD takes in light at frame number 4, auxiliarylight has already been emitted, so that, by an operation flow indicatedby arrow C, the exposure amount is computed (at frame number 4) {(3) theexposure amount after the emission of auxiliary light is determined}. Atthis time, since auxiliary light is emitted, the luminance level of animage is increased. At this time, by selecting the AF frame and making adetection, the luminance level is detected to compute the exposureamount.

When the amount of change in the exposure due to the emission ofauxiliary light is greater than a predetermined threshold value, theexposure amount is limited so that a sudden change in the luminancelevel of the image is restricted. This limiting operation is carried outwhen the amount of change in the exposure exceeds a predeterminedthreshold value. Here, the predetermined threshold value is previouslyset in RAM (not shown) in the exposure control circuit 9. By anoperation flow indicated by arrow D, the exposure amount that has beencomputed in this way is reflected at frame number 7 where the CCD takesin light {(4) the exposure amount is reflected}. At frame number 7 wherethe CCD takes in light, by setting the exposure amount after theemission of auxiliary light, the object has sufficient contrast, so thatthe precision of the auto-focusing can be increased.

Even if the exposure amount after the emission of auxiliary light isstill not the proper value, by repeating these steps, the precision ofthe exposure is increased, so that the precision of the auto-focusingcan be increased even more.

FIG. 3 is a flowchart showing the flow of the steps of the exposurecontrolling operation of the digital camera.

In FIG. 3, first, the illuminance of outside light is detected by thedigital camera (Step S1), and a determination is made as to whether ornot auxiliary light is required (Step S2). When it is determined thatauxiliary light is required (when the answer is “yes” in Step S2), theCCD takes in a detected value based on the exposure amount that has beencomputed at a previous frame (Step S3), and the image signal processingcircuit 6 makes a computation (Step S4). Thereafter, auxiliary light isemitted (Step S5), and the exposure amounts at the AE frame and the AFframe are computed (Step S6).

Next, in Step S7, a determination is made as to whether or not theexposure amount of the AF frame has markedly increased compared to theexposure amount of the AE frame. When it has been determined that theexposure amount of the AF frame has not markedly increased, “AF frameexposure” is selected (Step S8). When it has been determined that theexposure amount of the AF frame has markedly increased, “AE frameexposure” is selected (Step S9), and the operation proceeds to Step S10.In Step S10, a determination is made as to whether or not the amount ofchange in the exposure has exceeded a threshold value. When it has beendetermined that the amount of change in the exposure has exceeded thethreshold value, the exposure amount is limited (Step S11). Theoperation proceeds to Step S12. On the other hand, when it has beendetermined that the amount of change in the exposure has not exceededthe threshold value in Step S10, the operation proceeds to Step 12without limiting the exposure amount.

Next, in Step S12, the CCD 3 takes in the detected value of the AFframe, and the image signal processing circuit 6 makes a computation(Step S13). Thereafter, the exposure amounts of the AE frame and the AFframe are computed (Step S14).

Next, in Step S15, a determination is made as to whether or not theexposure amount of the AF frame has markedly increased compared to theexposure amount of the AE frame. When it has been determined that theexposure amount of the AF frame has not markedly increased, “AF frameexposure” is selected (Step S16). When it has been determined that theexposure amount of the AF frame has not markedly increased, “AE frameexposure” is selected (Step S17), and the operation proceeds to StepS18. In Step S18, a determination is made as to whether or not theamount of change in the exposure has exceeded the threshold value. Whenit has been determined that the amount of change in the exposure hasexceeded the threshold value, the exposure amount is limited (Step S19).The operation proceeds to Step S20 to determine whether or not theamount of change in the exposure is a proper amount. On the other hand,when a determination is made that the amount of change in the exposurehas not exceeded the threshold value in Step S18, the operation proceedsto Step 20 without limiting the exposure amount.

In Step S20, when the amount of change in the exposure is not the properamount, the operation returns to Step S12, and the Steps S12 to S20 arerepeated. In Step S20, when the amount of change in the exposure is theproper amount, a proper exposure controlling operation is carried out inStep S21 based on the exposure amount obtained in Step S14.

On the other hand, when it is determined that auxiliary light is notrequired (when the answer is “no” in Step S2), the CCD 3 takes in adetected value (Step S22), and the image signal processing circuit 6makes a computation (Step S23). Thereafter, the exposure amount of theAE frame and the exposure amount of the AF frame are computed (StepS24), and, based on the exposure amounts obtained in Step S24, theexposure controlling operation is carried out (Step S25).

As described in detail above, according to the present invention,auxiliary light is emitted during auto-focusing to detect the luminancelevel of an object. Based on the luminance level, the exposure amount isdetermined. Based on the exposure amount, the exposure controllingoperation is carried out. Therefore, the exposure controlling operationis properly carried out, so that the precision of the auto-focusing canbe increased.

In addition, according to the present invention, auxiliary light isemitted during auto-focusing to detect the luminance level of an object.Based on the luminance level, the exposure amount is determined. Basedon the exposure amount, the exposure controlling operation is carriedout. Therefore, pictures can be taken in dark places.

Further, according to the present invention, a determination is made asto whether or not the exposure amount of the AF frame has markedlyincreased than the exposure amount of the AE frame. When it isdetermined that it has markedly increased in the determining step, theexposure controlling operation is carried out based on the luminancelevel of the AE frame. Therefore, it is possible to carry out theexposure controlling operation without putting the camera in the scan AFmode, so that the exposure controlling operation can be quickly carriedout.

When auto-focusing is carried out, auxiliary light is emitted to detectthe luminance level of an object. Therefore, even if a userintentionally sets the exposure amount outside the range of properexposure amounts in, for example, manual mode, the user can effectivelycarry out auto-focusing by detecting the luminance level of the objectagain.

1. A method for controlling exposure of a digital camera comprisingmeans for emitting auxiliary light, the method comprising the steps of:detecting the illuminance of outside light; determining whether or notthe auxiliary light is required based on the detected illuminance of theoutside light; prior to emitting the auxiliary light, detecting theluminance level of an object in an AE frame in a monitoring mode underthe outside light; after emitting the auxiliary light, detecting againthe luminance level of the object in an AF frame in a scan AF mode underthe auxiliary light; determining whether or not the exposure amount ofthe AF frame has increased markedly than the exposure amount of the AEframe; and when it has been determined that the exposure amount of theAF frame has increased markedly in the determining step, performing anexposure controlling operation based on the luminance level in the AEframe, or, when it has been determined that the exposure amount of theAF frame has not increased markedly in the determining step, performingan exposure controlling operation based on the luminance level in the AFframe.
 2. A method for controlling exposure of a digital cameraaccording to claim 1, wherein the exposure controlling step comprisescontrolling the opening ratio of an aperture, the electronic shutteringamount of an imaging device, and the gain of an imaging signal outputfrom the imaging device.
 3. A method for controlling exposure of adigital camera according to claim 1, wherein, when the ratio between theexposure amount after the emission of the auxiliary light and theexposure amount prior to the emission of the auxiliary light exceeds apredetermined threshold value, the exposure amount in the AF frame islimited.
 4. An exposure controller of a digital camera comprising meansfor emitting auxiliary light, the exposure controller comprising: meansfor detecting the illuminance of outside light; means for determiningwhether or not the auxiliary light is required based on the detectedilluminance of the outside light; means for detecting the luminancelevel of an object in an AE frame in a monitoring mode under the outsidelight prior to emitting the auxiliary light, and for detecting again theluminance level of the object in an AF frame in a scan AF mode under theauxiliary light after emitting the auxiliary light; means fordetermining whether or not the exposure amount of the AF frame hasincreased markedly than the exposure amount of the AE frame; and meansfor performing an exposure controlling operation based on the luminancelevel in the AE frame when it has been determined that the exposureamount of the AF frame has increased markedly in the determining step,or based on the luminance level in the AF frame when it has beendetermined that the exposure amount of the AF frame has not increasedmarkedly in the determining step.
 5. An exposure controller of a digitalcamera according to claim 4, wherein the exposure controlling meanscontrols the opening ratio of an aperture, the electronic shutteringamount of an imaging device, and the gain of an imaging signal outputfrom the imaging device.
 6. An exposure controller of a digital cameraaccording to claim 4, wherein, when the ratio between the exposureamount after the emission of the auxiliary light and the exposure amountprior to the emission of the auxiliary light exceeds a predeterminedthreshold value, the exposure amount in the AF frame is limited.